[TowerTalk] One more dipole-balun question

[Jim Lux]

On 6/3/13 6:32 AM, John Geiger (AF5CC) wrote:
> I had previously said that I had a G5RV type dipole that I made
> myself.  After repairing it today following a run in with a tree
> branch last Wednesday, I realize that it isn't quite that.  It is
> more of an "off center fed dipole". It isn't the classic "Windom off
> center fed" where one leg is twice as long as the onther.  On my
> antenna one leg is approximately 50 feet long, and the other leg is
> around 60 to 65 feet.  It is fed with 300 ohm radio shack twin
> lead-about a 25 foot run, which goes to a 4:1 voltage balun, and then
> to 75ohm RG-11 coax to the shack.
>
> Given this new revelation, that it is a off center fed dipole, what
> is now the best balun for the twin lead to coax junction?  I am
> assuming that it isn't quite a balanced antenna, since the legs
> aren't equal, so how crucial does a balanced to unbalanced (coax
> feedline) transformer become?  My main interest is trying to keep the
> RF off of the coax shield, although I haven't had any real problems
> with RFI in the shack.
>

You've got two separate issues going on here: one is that an OCF dipole 
has a feedpoint impedance that's higher than a center fed.  So you 
probably want some sort of transformer.  You'd have to model your 
specific lengths and feedpoint position at the frequencies you operate 
at to find what ratio you want.  4:1 and 9:1 are common, because the 
first is a 2:1 turns ratio transformer, the second is 3 turns on the 
High Z side and 1 turn on the Low Z side.

Do you have a way to measure the antenna Z?

The other is that the asymmetric antenna will tend to induce currents on 
the ouside of the feedline (or common mode currents on a balanced line). 
  As a practical matter, most symmetric dipoles have this problem too: 
rare is the installation in the middle of a perfectly open field for 
100s of meters on every side with the feedline perfectly symmetric to 
the dipole elements on a windless day.

The common mode/outside the coax current issue has an easy fix: choke it 
by making a lossy inductor with the right cores (31 mix is favored for 
this application).


you do NOT want to use the same kind of core for the transformer as for 
the RF choke.  The RF choke core should be one that shows a lot of loss: 
the exact wrong choice for the transformer core.